A sealing element of the above type having a hard material layer which comprises a hard layer component applied to the working surface of the shut-off body and a subsequent covering layer component and the associated plasma CVD coating process for applying the hard material layer to the working surface of the shut-off body is known from DE-A 38 32 692.
According to this document, shut-off bodies are, to coat their working surfaces, placed in the coating chamber on a specimen holder having a negative potential relative to the plasma. For initial physical etching of the working surfaces of the shut-off bodies with argon, the coating unit is first operated as cathode atomization (sputtering) unit. For subsequent deposition of the bonding layer component of the hard material layer on the etched working surface, the same unit is then operated under certain first process parameters as high-frequency plasma CVD unit, with the argon gas in the coating chamber being replaced, for example, by tetramethylsilane. With altered second process parameters, the covering or sliding layer component is then deposited from a gas mixture of tetramethylsilane and hexane after deposition of the bonding layer component.
The sealing elements obtained by this method (these can advantageously be coated ceramic plates for water valves, e.g. single-hand mixers) have, thanks to the hard material layer applied, sufficiently low coefficients of sliding friction and static friction when sliding over one another, even in the presence of water, so that greasing of the sealing surfaces as has hitherto been required for uncoated ceramic plates is no longer necessary.
A disadvantage of the known sealing elements, which are coated with a single-layer and multilayer hard material layer, is that their hot-water resistance is unsatisfactory. This is the case particularly when a hot-water resistance of more than 1000 operating hours is to be achieved without the hard material layer applied becoming detached from the shut-off body.